The present invention relates to sabot-launched/fin-stabilized kinetic energy projectiles. More particularly, the present invention relates to a sabot having a stiffener for a kinetic energy projectile with a penetrator of a type that is launched at high muzzle velocity.
A typical fin-stabilized kinetic energy projectile consists of a rod-type penetrator, a penetrator cap at the front end and a fin assembly at the rear end. The sabot tightly clamps about the kinetic energy projectile to seal against the passage of explosive gasses by sealing against the kinetic energy projectile and against the gun bore. This permits the stabilizing fins to pass through the gun bore without contact therewith. Just after the kinetic energy projectile and sabot leave the gun bore, aerodynamic forces strip the sabot from the kinetic energy projectile. The kinetic energy projectile continues toward the target. Wind resistance, which reduces the velocity, is decreased by discarding the sabot.
When launched, the kinetic energy projectile possesses kinetic energy according to the relation KE=(1/2)mv.sup.2, where the velocity v is typically in the range between about 1500 m/s to about 2000 m/s. Thus, a kinetic energy projectile with mass m achieves its maximum kinetic energy by being launched at the greatest possible velocity. The penetrator is made of a heavy metal, such as tungsten or depleted uranium, in order to give it as great a mass as possible. Since the force available to launch the kinetic energy projectile is a constant, reducing the mass of the system in the gun bore (kinetic energy projectile plus sabot) increases the velocity and increases the total kinetic energy of the kinetic energy projectile.
A sabot is used to cause the kinetic energy projectile to maintain its proper place and position in the bore of a gun when launched. Since the diameter of a kinetic energy projectile is much smaller than the diameter of the gun bore in which it is used, a sabot is sized to the diameter of the gun bore. A sabot holds a kinetic energy projectile as the sabot and kinetic energy projectile together are launched through the gun bore and separates from the kinetic energy projectile immediately after the kinetic energy projectile leaves the gun barrel.
A problem encountered in the launching of fin-stabilized kinetic energy projectiles with long penetrators at high muzzle velocities is the presence of aiming dispersion between successive rounds. That is, successive kinetic energy projectiles will not hit the same target location, but will impact in a scattered pattern around a nominal location on the target. One factor causing this scattered pattern is in bore flexing of the kinetic energy projectile, which permits successive kinetic energy projectiles, upon emerging from the bore, to fly in slightly different initial directions. One reason for the existence of this flexing is that fin-stabilized kinetic energy projectiles typically have a length-to-diameter ratio of about 20 or greater. A length-to-diameter ratio of this magnitude creates a tendency for the kinetic energy projectile to flex in "limber-rod" fashion, absent some means of stabilization.
A second reason for the existence of this flexing is the presence of a clearance between the sabot and the gun barrel. The sabot cannot have exactly the same diameter as the gun bore. Machining tolerances and continued launching of kinetic energy projectiles from the gun bore also add to the slight clearance between the sabot and the bore. In order to reduce the sabot mass as much as possible, the sabot diameter is reduced a great deal at its longitudinal center, while maintaining contact with the gun bore toward its ends. This clearance between the sabot and the gun bore allows the kinetic energy projectile to bend, albeit slightly, within the gun bore while it is being launched at high velocity. The angular velocity and an associated flexing frequency or bending frequency imparted to the kinetic energy projectile and its sabot as a result of its buffeting balloting within the gun bore results in high angular and lateral impulse forces and moments on the kinetic energy projectile as it exits the gun bore. The kinetic energy projectile exits the gun bore in a direction slightly biased in a random direction from that indicated by the axis located through the center of the gun bore. Aiming dispersion between successive rounds results because each kinetic energy projectile exits the gun in a slightly different direction and with a slightly different angular orientation and angular acceleration, and thus flies a slightly different path, with the path differences being substantially randomly arranged about the path which would be taken in the absence of kinetic energy projectile flexing.
A solution to the aiming dispersion problem entails minimizing the in-bore flexing of the kinetic energy projectile by stiffening its sabot. One way of stiffening a sabot is to make it from a stiffer material, such as steel. Another way of stiffening a sabot is to make it from a lightweight material, such as aluminum or. metal-matrix composite, but to make it heavier and thicker by using greater amounts of material. This approach was described in technical report BRL-TR-3359, written by Kaste & Wilkerson of the U.S. Army Ballistic Research Laboratory and published in June, 1992. The authors conducted an analytical study on the effect of stiffening the sabot of the XM900E1 kinetic energy projectile through the use of a heavier and thicker aluminum sabot. Their results show that the yaw of the kinetic energy projectile reduces by a factor of one-half when the heavier and stronger sabot is used.
Both solutions, however, result in a degradation of the performance of the penetrator since, because the energy contained in the propellant used to launch the kinetic energy projectile is constant, using the heavier sabot reduces the velocity of the kinetic energy projectile, thereby imparting a lower kinetic energy to the penetrator.
The prior art sabot manufacturing method has high production costs. The method uses three or four circular cylindrical sections, or sabot petals, which are assembled into a cylinder. The assembled cylinder is then machined to the specified sabot shape. Kinetic energy projectiles typically employ four or six fins. If three sabot petals are used, each section covers 120.degree. of arc; if four petals are used, each section covers 90.degree. of arc. Sabot petals are used instead of one solid cylinder so the sabot is almost immediately separated by aerodynamic forces after the kinetic energy projectile leaves the gun bore. During sabot discard, the sabot petals are stripped away from the kinetic energy projectile by aerodynamic forces generated at an air scoop. The sabot petals avoid the stabilizing fins as the sabot is discarded.
The sabot petals are cut from a solid cylinder, but portions of cylinder material are inevitably removed during the cutting process. The solid cylinder must thus have a greater outside diameter than the maximum sabot diameter so that the diameter of the assembled cylinder approximates the sabot diameter. Further machining is required when the assembled cylinder is formed since its outside curve is no longer exactly circular due to the cutting process.